1. Field of the Invention
This invention relates to polycarbonates and more particularly to polycarbonates having flame retardant characteristics.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded products where impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
However, these polymers exhibit a brief though definite burning time when contacted with an open flame and do not meet a specified requirement for flamability resistance in applications where high temperatures and/or exposure to fire may be encountered. In addition, stabilizers or other functional additives which are normally used in polycarbonates will further modify the burning characteristics of the plastic, to make them unacceptable where a certain flame retardance requirement is specified.
Also, stabilizers and functional additives such as monomeric phosphates, phosphoric acid esters and thiophosphoric acid esters containing halogenated alkyl radicals have been incorporated into polymers to increase their flame resistant properties. Metal salts have also been utilized to provide flame retardance characteristics to polycarbonates. Further, in aromatic polycarbonate resins some of the phenolic diols used in the production thereof have chlorine or bromine atoms substituted on the aromatic ring to provide flame resistant characteristics to the final polycarbonate.
However, these stabilizers and halogenated phenolic diols have to be employed in such large quantities in order to obtain the flame resistant characteristics that they reduce the desirable physical properties of the polymers such as impact strength and even cause considerable deterioration in other physical properties such as resistance to hydrolysis. A particular disadvantage in using the halogenated diols in the synthesis of the polycarbonate is the increased cost of the final resin to an economically undesirable extent.
Illustrative of the techniques utilized to provide flame retardant characteristics to polycarbonates is U.S. Pat. No. 3,475,372. Therefore, in accordance with the present invention, self-extinguishing and flame resistant polycarbonates are provided which are substantially unmodified in physical properties.